1. Field of the Invention
The present invention relates to a multipole field-producing apparatus in a charged-particle optical system and, more particularly, to a multipole field-producing apparatus adapted to be used in an aberration corrector built in an electron microscope.
2. Description of Related Art
Various apparatus for correcting various aberrations in charged-particle optical systems have been proposed. For example, in a known apparatus for correcting spherical aberration in axisymmetric lenses in an electron microscope, two axisymmetric lenses are disposed between two multipole elements that produce hexapole fields. FIG. 3 schematically shows the structure of the illumination system of an electron microscope equipped with the prior art spherical aberration corrector. In the figure, the deflection system and parts of the focusing system are omitted. An electron beam 2 from a source 1 passes through a condenser lens 4 having an aperture 3. The beam made parallel to the optical axis enters spherical aberration correction optics 5. The beam emerging from the correction optics 5 and traveling parallel to the optical axis is directed at a specimen 7 through an objective lens 6.
The spherical aberration correction optics 5 has axisymmetric lenses 10 and 11 disposed between multipole elements 8 and 9 producing hexapole fields. The poles of the multipole elements 8 and 9 are so arranged that they are in phase with respect to the optical axis and that there is no rotational relationship about the optical axis within a plane perpendicular to the optical axis. The lenses 10 and 11 have the same focal length f, Spherical aberration is corrected when the distance between the multipole element 8 and the axisymmetric lens 10 is f, the distance between the axisymmetric lenses 10 and 11 is 2f, the distance between the axisymmetric lens 11 and the multipole element 9 is f, and the multipole elements 8 and 9 have the same excitation strength K and the same width Z (size) in the direction of the optical axis.
A given voltage or current is supplied to each pole of the multipole elements 8 and 9 and to the lenses 10 and 11 from a power supply 13 to correct spherical aberration appropriately. The power supply 13 is controlled by a control unit 14 that determines the value of voltage or current necessary for correction such that the spherical aberration correction optics 5 are controlled via the power supply 13.
In correction of spherical aberration using multipole elements such as hexapole elements, it is desired to produce only the hexapole field to correct the spherical aberration, i.e., to achieve the proper purpose. To produce the hexapole field using hexapole elements, the strength of the hexapole field can be set to any arbitrary value if the strengths of the various poles are set such thatIk=A cos(kπ)  (1)where Ik (k=1, 2, 3, . . . , 6) is the strength of each pole of each hexapole element. In this case, however, the phase angle is fixed, and the parameter controlling the strength is A.
When the hexapole field is produced according to Eq. (1) above, in actual instrumentation, however, deflecting fields (dipole fields, such as positional deviation of the image and coma) and quadrupole fields (astigmatism) are concomitantly produced due to mechanical accuracy at which electrodes or polepieces of the multipole elements forming the apparatus are machined or assembled.
In the past, unwanted aberrations, such as deflecting fields and quadrupole fields, produced concomitantly as described above have been removed as follows. First, the desired hexapole field is produced by setting the parameter A in Eq. (1) above to a desired value. Then, the unwanted aberrations, such as deflecting and quadrupole fields, are detected by some aberration detection means. Based on the results, deflecting and quadrupole fields for canceling the unwanted aberrations, such as deflecting and quadrupole fields, are intentionally produced according to Eq. (2).
                              I                      k            +                          =                                            C              1                        ⁢                          cos              ⁡                              (                                                      θ                    1                                    +                                                            k                      ⁢                                                                                          ⁢                      π                                        3                                                  )                                              +                                    C              2                        ⁢            cos            ⁢                                                  ⁢            2            ⁢                          (                                                θ                  2                                +                                                      k                    ⁢                                                                                  ⁢                    π                                    3                                            )                                                          (        2        )            where Ik+ is the strength of each pole added to Ik of Eq. (1). That is, it is possible to cancel deflecting and quadrupole fields produced concomitantly as control is provided to correct spherical aberration using Eq. (1), by adding Eq. (2) to Eq. (1). In the right side of Eq. (2), C1 and θ1 are strength and phase angle, respectively, for correction of deflection fields, and C2 and θ2 are strength and phase angle, respectively, for correction of quadrupole fields.
In the above description, the hexapole field uses hexapole elements. The phase angle of the hexapole field is constant. In actual instrumentation, a dodecapole (12-pole) element, for example, is used to obtain arbitrary phase angle and strength. In the case where the dodecapole element is used, it can be considered that two hexapole elements different in phase angle by 30 degrees are arranged within the same plane. Of course, if such a dodecapole element is used, unwanted aberrations, such as deflecting and quadrupole fields, are similarly produced concomitantly due to dimensional accuracies of components.
Background material is disclosed in Japanese Patent Laid-Open No. 2003-92078, “Spherical Aberration Corrector for Electron Microscope” and Japanese Patent Laid-Open No. 2003-157785, “Charged-Particle Beam Apparatus Equipped with Aberration Corrector”.
Where a desired multipole field, such as a hexapole field, is produced using a multipole element, such as a dodecapole element, if unwanted multipole fields, such as deflecting and quadrupole fields, are concomitantly produced due to inaccuracy at which parts are machined or assembled as described above, canceling the unwanted multipole fields using Eq. (2) above constitutes one method. However, it is not easy to implement this method, i.e., unwanted aberrations, such as deflecting and quadrupole fields, are detected by some method and four parameters C1, θ1, C2, and θ2 included in Eq. (2) are correctly determined.